1. Field of the Invention
The present invention relates to an organic light emitting element (referred hereinafter as “OLED element”) with improved light output rate, power consumption and lifespan and organic light emitting device employing the OLED element, as well as a method of manufacturing such OLED element and device.
2. Discussion of Related Art
An organic light emitting diode device (sometimes referred to as OLED device) is a self-emitting display device. Unlike the liquid crystal display (LCD) display device, it does not require an additional light source, and thus the OLED device can have much thinner and lighter profile than the LCD display device. Not only is the OLED device consumes less power than the typical LCD based display device, it provides better color accuracy, faster response rate, greater viewing angle, and higher contrast ratio (CR). Accordingly, a lot of research has been performed into developing more efficient emissive materials and OLED structures.
An OLED device commonly includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. When current is applied, electrons flow from the cathode to the anode. That is, the cathode provides electrons into the electroluminescent layer (EL layer), while the anode removes electrons from the EL layer, leading to holes in the EL layer. The electrons provided from the cathode combine with the holes in the EL layer and create excitons. This process releases photons, and thus light is generated. Different types of emissive materials can be used to change the color of the light. The intensity of the light is controlled by the amount of current applied.
Some OLED device employs organic light emitting elements configured with different luminescent layer(s) to emit a designated color of light (e.g., red, green, or blue) at a corresponding sub pixel region. In such OLED device, each OLED element emits designated color without using color filter elements, high luminance can be obtained from the OLED element. However, this type of OLED device (referred hereinafter as “RGB type”) requires the OLED elements to be disposed according to the sub pixel design of the display device, thereby severely limiting its application.
Unlike the RGB type, some OLED device employs white light producing luminescent layer in conjunction with red, green and blue color filter elements at the red, green and blue sub pixel regions (referred hereinafter as “RGBW type”). This configuration makes it easy to fabricate the OLED device for a variety of applications.
There is a tradeoff between the color saturation, the driving current, the luminance and the lifetime of the OLED device. In case of RGBW type OLED, the luminance of the OLED element is sacrificed as the white light from the OLED element passes through the color filter elements. In order to compensate the reduced luminance, higher current density is provided. Such increase in current density, however, leads to shortened lifetime of OLED element and undesirable color shifts as the display device ages.
Further, the color coordinates of the white light emitted from the OLED element can be distorted if the luminance of a certain emission layer becomes lower than the luminance of the other emission layers or the color coordinate of a certain emission layer deviates from its initial preferred setting. For example, if the luminance of an EL layer that emits blue light is lower than that of the other EL layers that emit other colored light, the OLED element may not emit the white light with the ideal color coordinate.
The driving voltage of the OLED element can be adjusted to correct such color coordinate shift caused by a certain emission layer. However, not only is the algorithm for correcting color coordinates require very complicated arithmetic operations, this method often involves increasing the current level at the white sub pixel region as well as some of the other pixel regions in order to correct the overall color saturation. This results in increased power consumption of the entire panel and reduced lifespan of the EL layer. Using a low-power driving algorithm following the aforementioned calibration significantly complicates the overall operation. Even if such algorithms can be performed, other side effects may occur.
Accordingly, there remains a need for white OLEDs having excellent emission efficiency and long life-spans.